There is well known a control apparatus for a vehicular automatic transmission configured to selectively establish a plurality of shift positions by engaging respective combinations of two frictional coupling devices of a plurality of frictional coupling devices, with output hydraulic pressures of respective ones of a plurality of solenoid valves provided in a hydraulic control circuit. The above-indicated hydraulic control circuit may suffer from an abnormal output of any solenoid valve, which takes place due to an abnormal switching operation of the solenoid valve per se caused by sticking of a valve spool in the presence of a foreign matter, for example, a hardware abnormality such as disconnection or shorting of a wire used to apply an electric signal to the solenoid valve, and an abnormality of an electronic control device per se, which operates to determine a need for shifting the automatic transmission and generate the electric signal. Such an abnormal output of the solenoid valve may cause an engaging action of any frictional coupling device other than the frictional coupling devices normally required to be placed in the engaged state for establishing the selected shift position of the automatic transmission, namely, any frictional coupling device not required to be placed in the engaged state for establishing the selected shift position. In this event, one of the frictional coupling devices placed in the engaged state, which has the smallest torque capacity, is brought into a slipping state, with a result of establishing one of the shift positions with the engaging actions of the other two non-slipping frictional coupling devices. As shown in the example of FIG. 8 a situation is supposed where a 7th speed position is established by the engaging actions of clutches C2 and C3 which are performed at a maximum engaging hydraulic pressure in a normal state. If a clutch C1 not required to establish the selected 7th shift position is engaged due to an abnormal output of a solenoid valve SL1 in this example, the clutch C2 of the three clutches C1-C3 is brought into the slipping state, so that a 3rd speed position may be established by the engaging actions of the clutches C1 and C3. As a result, the automatic transmission is shifted down from the 7th speed position to the 3rd speed position, causing a comparatively high stepping ratio of the speed ratio (ratio of the speed ratios of the two shift positions=speed ratio of the lower-speed position/speed ratio of the higher-speed position), and a rise of the operating speed of an engine and deceleration of the vehicle by a braking torque of the engine, which lead to deterioration of durability of the engine and a power transmitting system, and reduction (deterioration) of drivability of the vehicle.
In view of a possibility of occurrence of the abnormal output of the solenoid valve described above, there has been proposed to provide the hydraulic control circuit with a fail-safe valve having a fail-safe function to prevent the engaging action of a frictional coupling device due to the abnormal output of the solenoid valve, by cutting off an oil passage provided to apply the output hydraulic pressure of the abnormal solenoid valve to the frictional coupling device, or to establish a fail-safe shift position so as to reduce the stepping ratio of the speed ratio (difference of the speed ratios), by switching the oil passages utilizing the output hydraulic pressure of the abnormal solenoid valve. On the other hand, it is desirable not to use the fail-safe valve, from the standpoint of needs for reducing the weight and size of the hydraulic control circuit and for reducing the number of the required components. Namely, it is desirable to provide a fail-safe function equivalent to that of the fail-safe valve, without using the fail-safe valve. Patent Document 1 discloses a control apparatus for a transmission configured to perform shifting actions using a hydraulic control circuit not provided with the fail-safe valve, which control apparatus determines, on the basis of amounts of electric current applied to solenoid valves, whether the number of the solenoid valves concurrently placed in the engaged state is larger than in the normal states of the solenoid valves, and cuts off the supply of an electric power to the irrelevant solenoid valve other than the solenoid valves required to be placed in the engaged state, if the number is larger than in the normal states, so that the control apparatus performs the fail-safe function equivalent to that of the fails-safe valve.